DISPERSAL OF MERCURY-CONTAMINATED SEDIMENTS BY GEOMORPHIC PROCESSES, SIXMILE CANYON, NEVADA, USA - IMPLICATIONS TO SITE CHARACTERIZATION AND REMEDIATION OF FLUVIAL ENVIRONMENTS

The discovery of the Comstock Lode near Virginia City, Nevada, in 1859 resulted in the construction of more than 30 stamp-mills along Sixmil e Canyon and its tributaries to process the gold and silver ore. Extra ction of the precious metals from the ore was accomplished using a cru de mercury (Hg) amalgamation process. As a result, a substantial quant ity of Hg was released along with tailings materials to this tributary of the Carson River. During the past 134 years, Hg-contaminated sedim ents have been eroded from the mill sites and transported downstream b y fluvial processes, thereby expanding the influence of Hg pollution. Geomorphic and geochemical data have been combined in this study to do cument the distribution, quantity, and physical dispersal of Hg-contam inated materials from Sixmile Canyon to the Carson River. These data s how that the influx of Hg to the Carson River has varied through time as a function of the erosional and depositional processes operating on the Sixmile Canyon Alluvial Fan located between the canyon and the Ca rson River channel; relatively high influx rates to the river occurred immediately after mining began and from approximately 1933 to 1948. H g-polluted sediments are located within discrete areas of the fan and comprise about 21% of the total active and relict fan surface. Mass ba lance calculations estimate that about 31,500 kg of Hg, 18,200 oz of A u, and 1,205,800 oz of Ag are contained within 710,700 m(3) of contami nated materials. If site remediation is conducted, extraction of Au an d Ag, which is worth about $ 12 million at current market prices, woul d greatly defray the costs of clean-up activities. The study also illu strates that the effects of Hg may be temporally and spatially displac ed from the period and location of milling activities. Thus, we conclu de that to accurately assess the site for remediation, an understandin g of the spatial and temporal variations in geomorphic dispersal proce sses is required.